The present invention relates to improvements in control systems for filling tanks with liquids and, while not necessarily so limited, relates to filling tank trucks employed in the transport of petroleum products. Loading of petroleum products into the tanks of transport trucks involves several aspects, which are not necessarily found in other tank loading operations. First, the tank is usually divided into a plurality of separate compartments. By industry standards, provision is made for loading tanks having up to six compartments, though four and five compartment tanks are most commonly employed.
In the usual course of loading a tank truck, it is driven to a loading station, or loading rack, where a hose connection is made between each of the compartments being loaded and a bulk storage tank. The loading rack operator then presets a metering device for the delivery of a specified amount of a particular type of product into that tank compartment. The remaining compartments of the tank are also loaded in similar fashion with the same, or different types of petroleum products in predetermined amounts, as desired.
In such loading operations, the primary concern is to avoid overfilling of a tank compartment and the resulting hazard of spilled petroleum products (herein referred to as fuels for the sake of brevity), as well as the environmental damage that can result from a spill. A related concern is the building of excessive vapor pressure in the tank compartment as it fills, which may cause damage to tank seals or other equipment. Additionally, when the tank compartment is being unloaded, there can also be the danger of creating an excessive vacuum in the tank compartment as the gaseous head above the fuel increases in volume.
The overfill hazard has long been recognized and many systems have been proposed and employed to prevent overfills. Typically, various forms of sensors have been provided to detect the rise of fuel to a given upper level in a tank compartment. These sensors provide an enabling or “permit” signal to the rack mounted, control system components. The permit signal is a required input for the loading of fuel. When the fuel level in a compartment exceeds the preset, upper limit at the location of the sensor, the sensor becomes wet, the permit signal is terminated, and the loading rack control system shuts off the flow of fuel to the tank compartment. One such overfill prevention system is disclosed in U.S. Pat. No. 5,515,890.
Additionally, sensors have been used to detect if a fuel compartment has been essentially drained, so that the compartment can accept a preset quantity of fuel corresponding to its rated capacity. Such sensors provide a “retain” signal if the compartment is not fully drained prior to being loaded. The “permit” signal has also been made dependent on the existence of retain signals so that a loading operation cannot be commenced unless all compartments are empty. Indicia lights have been provided to identify a compartment that has not been fully drained.
As a further safeguard, pressure switches have been used in the vapor return piping of a tank truck to sense pressure or vacuum formation in the tank compartments. As fuel is pumped into a compartment at the terminal, fuel vapors are displaced inside the compartment. These vapors are normally routed back to the fuel terminal, via the vapor recovery line. If the recovery line malfunctions, however, the buildup of pressure can cause damage to the compartment and tank trailer. An over-pressure condition when loading will then cause a shutdown of the terminal pumps. The pressure switch requires special installation and wiring in addition to that associated with conventional overfill sensors. In a similar manner, when dumping fuel at a customer site, an excessive buildup of vacuum can also cause damage. Typically, a single pressure switch is positioned in the vapor return piping, which is a common conduit shared among the several tank compartments in the tank truck, and connected to an auxiliary input of an on-board monitor.